"It's an extraordinarily difficult situation, but I think we should be engaged rather than disengaged."

The PM said it was "right" to have lifted the EU arms embargo because it suggested there was a "moral equivalence" between Assad and the moderate forces seeking a democratic future for Syria.

Asked whether he expected the US to start sending weapons to the opposition forces, Mr Cameron said: "That is a matter for President Obama, but what I know President Obama will do, and will continue to do, is to work with us, with the French and other powers in the Gulf to work for those millions of Syrians who want a peaceful and democratic future for their country."